An anisotropically conductive sheet is a sheet exhibiting conductivity only in its thickness-wise direction or having pressure-sensitive conductive conductor parts exhibiting conductivity only in its thickness-wise direction when it is pressurized in the thickness-wise direction. Since the anisotropically conductive sheet has such features that compact electrical connection can be achieved without using any means such as soldering or mechanical fitting, and that soft connection is feasible with mechanical shock or strain absorbed therein, it is widely used as a connector for achieving electrical connection between a circuit device, for example, a printed circuit board, and a leadless chip carrier, liquid crystal panel or the like in fields of, for example, electronic computers, electronic digital clocks, electronic cameras and computer key boards.
On the other hand, in electrical inspection of circuit devices such as semiconductor integrated circuit devices such as packaged ICs and MCMs, wafers, on which integrated circuits have been formed, and printed circuit boards, in order to achieve electrical connection between electrodes to be inspected formed on one surface of a circuit device, which is an object of inspection, and electrodes for inspection formed on the surface of a circuit board for inspection, it is conducted to cause an anisotropically conductive elastomer sheet to intervene between an electrode region to be inspected of the circuit device and an electrode region for inspection of the circuit board for inspection.
As such anisotropically conductive elastomer sheets, those of various structures have heretofore been known. For example, Japanese Patent Application Laid-Open No. 93393/1976 discloses an anisotropically conductive elastomer sheet (hereinafter referred to as “dispersion type anisotropically conductive elastomer sheet”) obtained by uniformly dispersing metal particles in an elastomer, and Japanese Patent Application Laid-Open No. 147772/1978 discloses an anisotropically conductive elastomer sheet (hereinafter referred to as “uneven distribution type anisotropically conductive elastomer sheet”) obtained by unevenly distributing conductive magnetic particles in an elastomer to form a great number of conductive parts extending in a thickness-wise direction thereof and an insulating part mutually insulating them. Further, Japanese Patent Application Laid-Open No. 250906/1986 discloses an uneven distribution type anisotropically conductive elastomer sheet with a difference in level defined between the surface of each conductive part and an insulating part.
In the uneven distribution type anisotropically conductive elastomer sheet, since the conductive parts are formed in accordance with a pattern antipodal to a pattern of electrodes of a circuit device to be connected, it is advantageous compared with the dispersion type anisotropically conductive elastomer sheet in that electrical connection between electrodes can be achieved with high reliability even to a circuit device small in the arrangement pitch of electrodes to be connected, i.e., center distance between adjacent electrodes.
However, the conventional anisotropically conductive elastomer sheets have involved the following problems.
In electrical inspection of circuit devices such as semiconductor integrated circuit devices, a burn-in test that electrical inspection of circuit devices is conducted under a high-temperature environment is carried out for the purpose of sorting circuit devices having latent defects.
A burn-in test using an anisotropically conductive elastomer sheet will be specifically described. A circuit device to be inspected is arranged on one surface of the anisotropically conductive elastomer sheet, a circuit board for inspection is arranged on the other surface of the anisotropically conductive elastomer sheet, and they are pressurized in a thickness-wise direction thereof, whereby electrical connection between electrodes to be inspected of the circuit device to be inspected and inspection electrodes of the circuit board for inspection is achieved. The circuit device to be inspected is then heated to a prescribed temperature, and held for a prescribed period of time in this state. Thereafter, necessary electrical inspection of the circuit device to be inspected is performed.
Since an uncured low-molecular weight component remains in an elastic polymeric substance for forming the anisotropically conductive elastomer sheet, for example, silicone rubber, and the low-molecular weight component bleeds out to the surface of the anisotropically conductive elastomer sheet, however, the circuit device to be inspected coming into contact with the anisotropically conductive elastomer sheet is contaminated.
In addition, since the anisotropically conductive elastomer sheet bears adhesiveness at a high temperature because the low-molecular weight component remains therein, the anisotropically conductive elastomer sheet adheres to the circuit device to be inspected when it is held for a long period of time in a state brought into contact under pressure with the circuit board to be inspected under a high-temperature environment. As a result, a problem that one or both of the anisotropically conductive elastomer sheet and the circuit device to be inspected are damaged when both are separated from each other is involved.
On the other hand, when the burn-in test is conducted by causing a sheet-like connector to intervene between the anisotropically conductive elastomer sheet and the circuit device to be inspected, the contamination of the circuit device to be inspected with the low-molecular weight component and the adhesion of the anisotropically conductive elastomer sheet to the circuit device to be inspected can be avoided.
In such a case, however, the anisotropically conductive elastomer sheet adheres to the sheet-like connector. As a result, one or both of the anisotropically conductive elastomer sheet and the sheet-like connector are damaged when both are separated from each other. In addition, when the anisotropically conductive elastomer sheet adheres to the sheet-like connector, the sheet-like connector causes bent. When the sheet-like connector is used in electrical inspection of a circuit device in this state, it is thus difficult to achieve stable electrical connection to all electrodes to be inspected in the circuit device to be inspected.
Further, when a conventional anisotropically conductive elastomer sheet is used in electrical inspection of a circuit device having electrodes to be inspected, which are formed of solder, the following problem is involved.
Namely, when the anisotropically conductive elastomer sheet is used repeatedly over many times in a probe test of a circuit device having electrodes to be inspected, which are formed of solder or used repeatedly in a test under a high-temperature environment, for example, a burn-in test, a solder material forming the electrodes to be inspected adheres to the surface of the anisotropically conductive elastomer sheet and further diffuses into conductive particles. As a result, it is difficult to retain necessary conductivity.
In order to solve such a problem, there has been proposed a means for forming a metal layer having diffusion resistance to the solder material on the surfaces of conductive parts in the anisotropically conductive elastomer sheet (see, for example, Japanese Patent Application Laid-Open No. 2002-280092).
However, it is impossible to inhibit the adhesion of the solder material to the surface of the metal layer by such a means. After all, it is thus difficult to retain necessary conductivity over a long period of time.
Further, the conventional uneven distribution type anisotropically conductive elastomer sheets involve the following problem.
Since an insulating part of a considerably wide area is present in the uneven distribution type anisotropically conductive elastomer sheet, the surface of the insulating part of the uneven distribution type anisotropically conductive elastomer sheet is charged with static electricity according to usage or service environment of the uneven distribution type anisotropically conductive elastomer sheet, so that various problems arise.
For example, when the uneven distribution type anisotropically conductive elastomer sheet is used in electrical inspection of a circuit device, electrical connection between the circuit device to be inspected and a circuit board for inspection is achieved by causing this anisotropically conductive elastomer sheet to intervene between the circuit device to be inspected and the circuit board for inspection and pressurizing the anisotropically conductive elastomer sheet, thereby performing the electrical inspection. However, electric charge is easy to be generated by a pressurizing operation and a separating operation. Therefore, the electric charge is accumulated on the surface of the insulating part in the anisotropically conductive elastomer sheet when the electrical inspection of a great number of circuit devices is continuously conducted, whereby the surface of the insulating part comes to be charged with high-voltage static electricity.
When the static electricity is discharged through the conductive parts of the anisotropically conductive elastomer sheet, an adverse influence may be exerted on not only the conductive parts of the anisotropically conductive elastomer sheet and a wiring circuit of the circuit board for inspection, but also the circuit devices, which are objects of the inspection, in some cases. As a result, there is a possibility that the anisotropically conductive elastomer sheet and the circuit board for inspection may be damaged, or the circuit devices to be inspected may be broken.